THE PINEAL BODY 239 



ment of the pineal body follows the general lines of glandular 

 differentiation. The pineal body is, therefore, a glandular 

 structure and as such, is necessary in some way to metabolism. 



III. The histology of the organ gives clear evidence that the 

 epiphyseal complex of vertebrates possesses a pluripotentiality 

 whose fundamental inherent tendency is in the interest of glandu- 

 lar differentiation, but in a few instances, as in cyclostomes,. 

 amphibia, and in primitive reptiles, the pineal organ may become 

 further differentiated in the interest of a highly specialized 

 sensory mechanism which has, or has had, visual function. As 

 a gland, it may in some cases, contribute its secretion to the 

 cerebrospinal fluid, but in the higher vertebrates, as in ophidians, 

 chelonians, birds, and mammals, it is an endocrinic organ, 

 contributing the products of its secretion to the blood stream. 



IV. a. There is no direct relation between the parietal eye and 

 the pineal body, but each is of itself an adaptive modification 

 answering the demands for, or representing, an inherent impulse 

 toward the development of a parietal eye, on the one hand, or a 

 glandular organ, on the other. 



6. The pineal body as it appears in mammals cannot be 

 regarded as the vestigial or metamorphosed degenerated or 

 atrophic residuum of the parietal eye in vertebrates. 



V. The phylogenetic significance of the parietal eye in verte- 

 brates as the homologue of the median eye in invertebrates 

 should be accepted with much reservation. Until such time as 

 the homology between the vertebrate pineal region and some 

 corresponding area of the invertebrate brain is much more firmly 

 established than at present, the parietal eye as an index in the 

 evolution of the vertebrates from the invertebrates has but little 

 value. 



The authors desire to acknowledge their great indebtedness 

 and to express their appreciation to Professor George S. Hunt- 

 ington for his assistance in the preparatine of this monograph. 

 They also wish to express their thanks to Professor M. Allen 

 Starr for his liberality in supplying the means which have made 

 publication possible. 



